1. Field of the Invention
The present invention relates to a golf ball having a coating layer (a layer formed by coating) at its surface portion.
2. Related Art
Multi-piece golf balls such as two-piece golf balls and three-piece golf balls, and thread-wound golf balls are usually produced by a process which comprises enclosing a solid core or a thread-wound core with a cover material and forming dimples by compression or injection molding; applying a coating on the surface of the cover material; conducting mark stamping by a transfer printing method; and then forming an outermost layer coating of the ball. In some cases, the mark stamp may be directly applied to the cover material surface, and then an outermost layer coating may be formed. One-piece golf balls are produced by a process which comprises preparing a solid core having dimples by compression or injection molding, and then subjecting the solid core to mark stamping and the application of an outermost layer coating of the ball in this order.
In such conventional golf balls, the coating layer formed on the surface of the cover material and the coating layer formed as the outermost layer of the golf ball are very thin and their thicknesses are in the range of 15-35 .mu.m. The main reasons why the coating layers at the surface portion of the golf balls have been made thin are as follows:
(a) The coating layer formed on the cover material or formed as the outermost layer protects the golf ball from dirt or scratches and to improve the appearance thereof. For such purposes, it is sufficient for the coating layer to have a thickness of about 15-35 .mu.m. PA1 (b) When coating is performed after formation of dimples, the coating layer shallows the dimples and dulls dimple edges if formed relatively thick on the dimples. This impairs preciseness of the dimple shape. Accordingly, the thickness of the coating layer must be made as thin as possible in order to prevent the coating layer from impairing preciseness of the dimple shape.
However, the conventional golf balls in which the coating layer at the surface portion has a thickness of about 15-35 .mu.m have the following problems. That it, since the coating layer at the surface portion is thin, the surface coating hardly affects the characteristics of the ball. Therefore, the surface hardness of the golf ball depends on the hardness of the cover, and the surface friction coefficient of the golf ball, which correlates with the surface hardness of the golf ball, depends on the surface friction coefficient of the cover. Consequently, the surface friction coefficient of the golf ball becomes small when the hardness of the cover is high, while the surface friction coefficient of the golf ball becomes large when the hardness of the cover is low.
The cover of a golf ball is generally formed by directly injecting a cover material around a solid core or thread-wound core, or by enclosing the solid core or thread-wound core with previously injection-molded hemisphere cover materials (half cups), and conducting compression molding. Therefore, the lower limit of the thickness of the cover is about 1.2-1.3 mm, and it is difficult to form a cover having a thickness less than the limit through compression molding or injection molding.
Accordingly, in high-spin type golf balls with a cover which has a low hardness and a large surface friction coefficient (for example, a hardness of about 55 on the Shore D Scale), the core must be made hard in order to prevent the repulsiveness of the golf ball itself from decreasing due to its soft and thick cover. In this case, feel on impact becomes hard. In soft type golf balls having a relatively soft core, the cover must be made hard (for example, a hardness of about 65 on the Shore D Scale) in order to prevent the repulsiveness of the golf ball itself from decreasing due to its soft core. As a result, the surface friction coefficient of the golf ball decreases, so that only a low-spin performance can be obtained.
In short, in the conventional golf ball, since the coating layer is thin, the coating layer hardly affects the characteristics of the ball, so that the surface hardness and surface friction coefficient of the golf ball depends only on the surface hardness and surface friction coefficient of the cover. Therefore, the surface hardness and surface friction coefficient of a golf ball must be determined in the designing process, taking into consideration the overall structure of the golf ball and the relationship between the hardness of the core and the hardness of the cover, as in the above-mentioned examples of the high-spin type golf ball and the soft type golf balls. This decreases the degree of freedom in designing of the golf ball, especially, in designing of the surface hardness and the surface friction coefficient. As a result, it has been impossible to mitigate the problem of the high-spin type golf balls, i.e., hard feel on impact, or the problem of the soft type golf balls, i.e., low-spin performance.